1. Field of the Invention
This invention is generally related to an apparatus for measuring parameters associated with the flow of blood through a body segment. More particularly, this invention is related to an apparatus for monitoring the cardiac output by measuring blood flow in the thorax of the body of a human or an animal.
2. Description of the Related Art
Cardiac output is the volume of blood which the heart pumps in one minute and is one of the most important cardiovascular parameters. The cardiac output reflects the supply of oxygen and nutrients to tissue. Measurements of cardiac output provide invaluable clinical information for quantifying the extent of cardiac dysfunction, and for monitoring the operation of the heart during exercise, surgery, or the like.
Cardiac output may be measured either invasively or noninvasively. The invasive techniques for measuring cardiac output involve penetration of the skin by a catheter, require complex instrumentation which must be operated by skilled personnel, and present a risk to the patient. Invasive techniques such as indicator dilution and thermal dilution allow only intermittent measurement of cardiac output since it is possible to obtain only one determination of cardiac output per injection in dilution methods.
Noninvasive techniques for measuring cardiac parameters include ballistocardiography, electrical bioimpedance measurements, ultrasonics, phonocardiography, and vibrocardiography. The present invention is concerned primarily with the use of electrical bioimpedance measurements. Electrical bioimpedance measurements permit quantification of blood flow as a result of changes in electrical conductivity of a body segment. The electrical impedance technique for measuring cardiac output is based upon changes in thoracic electrical impedance caused by cardiovascular activity. A full and complete discussion of electrical bioimpedance measurements and instrumentation for performing the measurements is set forth in commonly assigned U.S. Pat. No. 4,450,527, issued on May 22, 1984, the disclosure of which is incorporated herein by reference.
As set forth in U.S. Pat. No. 4,450,527, electrical bioimpedance measurements are obtained by injecting a high frequency, low magnitude constant current through a segment of a patient's body by positioning a first current injecting electrode at one boundary of the body segment and a second current injecting electrode at a second boundary of the body segment. Of course, as set forth in U.S. Pat. No. 4,450,527, multiple electrodes may be advantageously used for each of the current injecting electrodes. Changes in the electrical bioimpedance of the body caused by blood flow in the defined body segment are detected by measuring a voltage developed across the body segment. This voltage is measured by a set of voltage sensing electrodes which are positioned on the body segment within the boundaries defined by the current injecting electrodes. Again, multiple electrodes can be used for each of the voltage sensing electrodes.
Typically, the body segment chosen for determining cardiac output is the thorax of the patient. Thus, as illustrated in U.S. Pat. No. 4,450,527, in an exemplary measurement procedure, a pair of upper sensing electrodes are attached to the patient's neck on opposite sides at the intersections of a line encircling the root of the neck with the frontal plane of the patient. A pair of upper current injectng electrodes are attached to the patient's neck approximately five centimeters above the upper sensing electrodes. A pair of lower thoracic anterior sensing electrodes are placed at the intercostal space at each midclavicular line at the xiphoid process level. A pair of posterior sensing electrodse are placed at the same level as the anterior sensing electrodes at the intercostal space at the midscapular line. A first pair of lower current injecting electrodes are located approximately five centimeters below the lower thoracic anterior sensing electrodes. A second pair of lower injecting electrodes are attached to the patient, approximately five centimeters below the posterior sensing electrodes. Typically, the electrodes are spot electrodes which are pregelled and are attached to the prepared skin of the patient.
The electrodes are electrodes connected by wires to a bioimpedance measurement apparatus, such as the apparatus described in U.S. Pat. No. 4,450,527. Other measurement devices, having varying degrees of sophistication and cost could also be used. For the purposes of this application, reference will be made to the apparatus described in U.S. Pat. No. 4,450,527.
As set forth in U.S. Pat. No. 4,450,527, the changes in the electrical bioimpedance of the patient can be continuously measured during exercise or other diagnostic activity to thereby determine the cardiac output of the patient. Several cardiovascular variables can be measured and displayed using the measurement device.
As one can envision, or readily observe by reviewing U.S. Pat. No. 4,450,527, a number of electrical interconnection wires and corresponding electrodes are required to make the thoracic electrical bioimpedance measurements. Thus, although the device described in U.S. Pat. No. 4,450,4527, and other such devices have substantial diagnostic uses, the use of electrodes applied to the skin and wires emanating from the electrodes may present significant problems if bioimpedance measurements are desired during thoracic surgery or upper abdominal surgery. During such surgery, when the patient is sterile and draped with surgical cloths, it is sometimes difficult to verify the correct and reliable connection of the surface electrodes. During the thoracic surgery, the metal retractors, used to hold the rib cage open, may alter the electrical field distribution and interfere with the accuracy of the data obtained by electrical bioimpedance measurements. Similarly, if a patient is in septic shock, wherein the thoracic wall is highly perfused with liquid, the increased conductivity of the superficial layers of the thoracic wall will affect the absolute value of the stroke volume data and may therefore prevent the maseurement of useful data. The primary source of impedance variation originates in the descending thoracic aorta; however, the effects of the changes in volume of the thoracic aorta are less pronounced while using surface electrodes because the variation of electrical bioimpedance of all other thoracic sources, such as the volume of the blood in the heart and in the pulmonary circulation, will get superimposed over the thoracic aorta bioimpedance variation, thus diminishing the resolution of the measurement. Therefore, a need exists for an apparatus for measuring thoracic bioimpedance changes that are more responsive to the volume of the blood in the thoracic aorta and which can be used during surgical procedures without interfering with the procedure and without being substantially affected by metal instruments used during the procedure.